While the great brains of physics will argue that a starfield seen from the perspective a superluminal traveller will not smear into little spectral streaks, the effects is ubiquitous enough in contemporary science-fiction and fantasy works that it has become a kind of visual staple. So, science be damned -- the following tutorial illustrates a simple way to quickly apply the rainbow-streak effect to your moving starfields.
I came upon this technique in the autumn of 2003 while producing my own independent animated short entitled Space Attack!
Generate Stars The first step is to generate a moving starfield. While there are plug-ins that can generate a passable starfield, I've chosen to build my own out of a cloud of little spheroids -- a static particle system output by PowerParticles Pro for ElectricImage Universe -- which I then drove my camera through.
Basic Stars Layer Streakiness is added by using Motion Blur. In this instance, I've used the unreal setting of 999 degrees in order to get the most streaking per frame. The output from this set-up is the Basic Stars layer.
Fog Stars Layer This additional layer contains only extreme foreground stars. To accomplish this, I used the fog-to-alpha feature in Universe, setting the parameters to fade out anything that wasn't in the immediate vicinity of the camera. The same effect could be achieved using fog-to-black, and then applying the layer as a luma stencil during compositing, if your 3D application doesn't support fogging to transparency. (Note: if you don't have this level of control or are using stock footage, you can try to isolate the foreground stars by gating the contrast to only capture the brightests objects in the clip -- usually the stars at their biggest as they whiz by camera.)
Offset Coloured Clones Next, in the compositing application of your choice (I use Adobe After Effects), make three copies of the Basic Stars layer. Tint one layer green, and leave it where it is. Tint one layer red, and offset the layer forward in time one frame. Tint the last layer blue, and offset the layer backward one frame in time. Apply the upper two layers with the Screen transfer mode for spectrum-like transitions on the layer overlaps.
Above: Tinted layers, offset one frame each
Below Left: Original layer, tinted; Centre: Add screen red, one frame behind; Right: Add offset screen blue, one frame ahead. Finishing (Basic) If you're just using the basic method, you're pretty much done. I like to add a layer of slowly rotating background stars to fill out the picture a bit, but it's a matter of taste.
Finishing (Advanced) For a slicker final product, use the Basic Stars layer as your feature, but accent it by applying streaks to the Fog Stars layers, instead. This way the stars only streak out into rainbows when they come very close to camera. To do this, simply offset and tint a duplicated Fog Stars layer instead of a Basic Stars layer. Presto!
That's it. Share and enjoy!
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